1. Technical Field
This invention pertains to semiconductor packaging in general, and in particular, to a method and apparatus for making semiconductor packages with stacked dies.
2. Related Art
The increasing demand for electronic devices that are smaller, lighter, and yet more functional has resulted in a concomitant demand for semiconductor packages that have smaller outlines and mounting footprints, yet which are capable of increased component packaging densities.
One approach to satisfying this demand has been the development of techniques for stacking the semiconductor dies, or xe2x80x9cchips,xe2x80x9d contained in the package on top of one another. Examples of die-stacking techniques may be found, for example, in U.S. Pat. No. 5,323,060 to R. Fogel, et al.; U.S. Pat. No. 5,815,372 to W. N. Gallas; Re. Pat. No. 36,613 to M. B. Ball; U.S. Pat. No. 5,721,452 to R. Fogel, et al.; and, Japanese Patent Disclosures 62-126661, 4-56262, 63-128736, and 10-256470.
FIGS. 1 and 2 are respectively a top plan and a cross-sectional side elevation view of a semiconductor package 10 incorporating a pair of stacked dies 14 and 16 in accordance with the respective methods and apparatus of the prior art. The package 10 illustrated is a ball grid array (xe2x80x9cBGAxe2x80x9d) type of package, so-called because of the balls of solder 19 formed on the bottom surfaces of the substrate 12, which function as input/output terminals of the package. The package 10 includes a conventional interconnective substrate 12 and a first semiconductor die 14 mounted on a top surface of the substrate. A second die 16 has been xe2x80x9cstacked,xe2x80x9d i.e., mounted, on top of the first die 14. The dies 14 and 16 typically include a plurality of input/output wire bonding pads 34 located at the peripheral edges of their respective top, or xe2x80x9cactive,xe2x80x9d surfaces.
The substrate 12 may comprise a flexible resin tape, a rigid fiber-glass/copper sheet laminate, a co-fired ceramic coupon, or a metal lead frame, all of known types in the industry, depending on the particular type of semiconductor package 10 at hand. The connective substrate 12 illustrated in the BGA package 10 shown in FIGS. 1 and 2 comprises a layer 20 (see FIG. 2) of an insulative material, e.g., a polyimide resin film, laminated between conductive layers 22, 24 of a metal, e.g., copper or aluminum, that comprise the respective top and bottom surfaces of the substrate.
The conductive layers 22, 24 are typically patterned, e.g., by photolithography and etching techniques, to define wire bonding pads 26 and circuit traces 27 in the top layer 22, and solder ball mounting lands 28 in the bottom layer 24. The terminal pads 26 and traces 27 are typically connected to the solder ball lands 28 through the thickness of the insulative layer 20 by xe2x80x9cviasxe2x80x9d 30, i.e., plated-through holes in the layers. Either or both of the conductive layers 22, 24 may be coated over with an insulative xe2x80x9csolder maskxe2x80x9d (not illustrated) that has openings in it through which the respective wire bonding pads 26 and/or solder ball lands 28 are exposed, and which serve to prevent bridging between the pads and/or lands by accidental solder splashes.
In an alternative embodiment, the substrate 12 may comprise a metal lead frame (not illustrated) having a die-mounting paddle centrally supported within a matrix of radially extending leads. In this embodiment, the dies 14 and 16 wire bond to inner ends of the leads of the lead frame, rather to bonding pads located on the substrate, and the formed leads serve as the input/output terminals of the package 10.
In the embodiment illustrated, the first die 14 is conventionally mounted to the top surface of the substrate 12 with, e.g., a layer of an adhesive or an adhesive film 13, and then electrically connected to the substrate by a plurality of fine, conductive wires 38, typically gold or aluminum, that connect the pads 34 on the die to the pads 26 on the substrate.
The second die 16 is mounted on the top surface of the first die 14 with an adhesive layer 15 comprising a second layer of an adhesive or a double-backed adhesive film that has a lateral perimeter 17 (shown by the dotted outline in FIG. 1) positioned entirely within the central area of the top surface of the first die and completely inside of the peripheral wire bonding pads 34 thereon. That is, the adhesive layer 15 does not contact or cover either the wire bonding pads 34 or the conductive wires 38 bonded thereto. The adhesive layer 15 positions the second die 16 sufficiently far above the first die 14 to prevent the former die from contacting the conductive wires 38 bonded to the latter die and shorting them out, and thus defines a peripheral space 19 (FIG. 2) between the two dies that extends around the entire perimeter 17 of the spacer. The second die 16 is then wire bonded to the substrate 12 in the same fashion as the first die 14. One or more additional dies (not illustrated) can then be stacked in tandem on top of the second die 16 using the same technique.
After the dies 14 and 16 are wire bonded to the substrate 12, the dies, substrate, and conductive wires 38 are xe2x80x9covermoldedxe2x80x9d with a dense, monolithic body, or xe2x80x9cmold capxe2x80x9d 60 (shown by dotted outline in FIG. 2, omitted for clarity in FIG. 1), of plastic, typically a filled epoxy resin, that encapsulates the packaged parts and protects them from environmental elements, particularly moisture.
In a stacked-die package 10 of the type illustrated in FIGS. 1 and 2, the dies 14 and 16 are wire bonded sequentially, typically with automat ed wire bonding equipment employing well-known thermal-compression or ultrasonic wire bonding techniques. As shown in FIG. 2, during the wire bonding process, the head 62 of a wire bonding apparatus applies a downward pressure on a conductive wire 38 held in contact with a wire bonding pad 34 on the die to effect a weld or bond of the wire to the pad.
Since the wire bonding pads 34 are located in the peripheral area of the respective top surfaces of the two dies, this entails the application, in the direction of the arrow shown in FIG. 2, of a relatively large, localized force to that area of the die. This does not present a problem with the bottom die 14, as it is supported from below by the substrate 12 and the adhesive layer 13. However, in the case of the second, top die 16, its peripheral portion is cantilevered out over the peripheral portion of the bottom die 14 by the adhesive layer 15, and is therefore unsupported from below. As a consequence, the top die 16 can crack or fracture during the wire bonding procedure, as illustrated in FIG. 2, which results in the entire assembly being scrapped.
Another problem that can result with the prior art die stacking techniques also relates to the peripheral space created between the opposing surfaces of the first and second dies 14 and 16 by the adhesive layer 15 and the plastic molding material used to form the body 60 that encapsulates the dies. In particular, the encapsulant material penetrates into the peripheral space during the molding process and forms a xe2x80x9cwedgexe2x80x9d between the two dies. If the encapsulant material has a different thermal coefficient of expansion than that of the adhesive spacer 15, it is possible for this wedge to expand within the peripheral space 19 with large changes in temperature of the package 10, and thereby fracture one or both of the dies 14 and 16, again resulting in a defective package that must be scrapped.
This invention provides a simple, inexpensive method for making a semiconductor package with stacked dies that eliminates fracturing of the dies during the wire bonding process or as a result of incompatible thermal expansions. The method permits the use of ultra-thin dies having the same size, and does not require the use of support pillars.
In one embodiment, the method includes the provision of a substrate, which may be either a conventional laminate or a lead-frame-type of substrate. A pair of semiconductor dies having the same size (e.g., identical dies), or at least the same length and width, are also provided. Each die has opposite top and bottom surfaces and a plurality of wire bonding pads located around the periphery of the top surface thereof. The bottom surface of the first die is attached to a top surface of the substrate, and the wire bonding pads on the first die are connected to wire bonding areas on the top surface of the substrate with a first plurality of conductive wires.
A measured quantity of an uncured, electrically non-conductive, fluid adhesive is dispensed onto the top surface of the first die. The adhesive is then squeezed, or distributed, laterally between the two dies and toward their respective peripheries by pressing the bottom surface of the second die down onto the adhesive until: 1) the bottom surface of the second die is spaced apart from the top surface of the first die by a layer of the adhesive thick enough to prevent shorting contact between the second die and the conductive wires bonded to the top surface of the first die; 2) each portion of the second die located below a respective one of the wire bonding pads on the top surface of the second die is supported from below by the layer of adhesive; and, 3) the wire bonding pads on the top surface of the first die and the inner ends of the conductive wires bonded thereto are encapsulated by the adhesive.
The adhesive is then cured, and the wire bonding pads on the top surface of the second die are wire bonded to wire bonding areas on the top surface of the substrate with a second plurality of conductive wires. The solidified layer of adhesive below the peripheral portion of the second die on which the wire bonding pads are located supports the die from below and thereby prevents the die from fracturing during the wire bonding process. In addition, because the adhesive substantially fills the peripheral space between the opposing surfaces of the two dies, it prevents the molding compound or other encapsulant formed over the stacked dies from flowing into the space and forming a potentially destructive xe2x80x9cthermal wedgexe2x80x9d between the dies. Further, since the adhesive 40 also covers the wire bonding pads 34 on the top surface of the first die 14, as well as the inner ends of the conductive wires 38 that are bonded thereto, it thereby helps to maintain the integrity of the electrical connection between the wires and the first die during the subsequent molding or other manufacturing processes.
A better understanding of the above and other features and advantages of the invention may be had from a consideration of the detailed description below of some exemplary embodiments thereof, particularly if such consideration is made in conjunction with the appended drawings.